LibWeb: Make Node::is_text() return true for CDATASection nodes

CDATASection inherits from Text, and so it was incorrect for them to
claim not to be Text nodes.

This fixes at least two WPT subtests. :^)

It also exposed a bug in the DOM Parsing and Serialization spec,
where we're not told how to serialize CDATASection nodes.

Spec bug: w3c/DOM-Parsing#38

Libraries/LibWeb/DOM/Node.h

@@ -110,7 +110,7 @@ class Node : public EventTarget {
 
     NodeType type() const { return m_type; }
     bool is_element() const { return type() == NodeType::ELEMENT_NODE; }
-    bool is_text() const { return type() == NodeType::TEXT_NODE; }
+    bool is_text() const { return type() == NodeType::TEXT_NODE || type() == NodeType::CDATA_SECTION_NODE; }
     bool is_document() const { return type() == NodeType::DOCUMENT_NODE; }
     bool is_document_type() const { return type() == NodeType::DOCUMENT_TYPE_NODE; }
     bool is_comment() const { return type() == NodeType::COMMENT_NODE; }

Libraries/LibWeb/DOMParsing/XMLSerializer.cpp

@@ -196,6 +196,14 @@ WebIDL::ExceptionOr<String> serialize_node_to_xml_string_impl(GC::Ref<DOM::Node
         return serialize_comment(static_cast<DOM::Comment const&>(*root), require_well_formed);
     }
 
+    // NOTE: CDATASection comes before Text since CDATASection is a subclass of Text.
+    if (is<DOM::CDATASection>(*root)) {
+        // Note: Serialization of CDATASection nodes is not mentioned in the specification, but treating CDATASection nodes as
+        // text leads to incorrect serialization.
+        // Spec bug: https://github.com/w3c/DOM-Parsing/issues/38
+        return serialize_cdata_section(static_cast<DOM::CDATASection const&>(*root), require_well_formed);
+    }
+
     if (is<DOM::Text>(*root)) {
         // -> Text
         //    Run the algorithm for XML serializing a Text node node.
@@ -220,12 +228,6 @@ WebIDL::ExceptionOr<String> serialize_node_to_xml_string_impl(GC::Ref<DOM::Node
         return serialize_processing_instruction(static_cast<DOM::ProcessingInstruction const&>(*root), require_well_formed);
     }
 
-    if (is<DOM::CDATASection>(*root)) {
-        // Note: Serialization of CDATASection nodes is not mentioned in the specification, but treating CDATASection nodes as
-        // text leads to incorrect serialization.
-        return serialize_cdata_section(static_cast<DOM::CDATASection const&>(*root), require_well_formed);
-    }
-
     if (is<DOM::Attr>(*root)) {
         // -> An Attr object
         //    Return an empty string.